1. Field of the Invention
The subject invention relates to thermosetting resin systems which contain certain secondary amine-terminated siloxane modifiers. The modified resins find uses as heat-curable matrix resins in fiber-reinforced prepregs, as laminating films, and as structural adhesives.
2. Description of the Related Art
Modern high-performance thermosetting resin systems contain a variety of heat-curable resins. Among these are epoxy resins, malemide-group-containing resins, and the cyanate resins. All these resins are noted for their high tensile and compressive strengths and their ability to retain these properties at elevated temperatures, and all find extensive use in the aerospace and transportation industries. Other thermosetting systems which may be useful at lower temperatures or for specific applications include the polyurethanes, polyureas, polyacrylics, and unsaturated polyesters.
Unfortunately, many of these resin systems tend to be brittle. Thus while exhibiting high strengths under constant or slowly changing stress/strain, these systems and the structures which contain them may be susceptible to impact-induced damage. It would be desirable to prepare matrix resin and adhesive formulations which maintain their high strength properties while having enhanced toughness.
In the past, functionalized elastomers such as the amino- or carboxy-terminated butadiene-acrylonitrile copolymers (ATBN and CTBN, respectively) available from B. F. Goodrich Corp. under the trademark Hycar.RTM. have been used with some degree of success in toughening both adhesive and matrix resin formulations. See, for example, the article by J. Riffle, et. al., entitled "Elastomeric Polysiloxane Modifiers" in Epoxy Resin Chemistry II, R. Bauer, Ed., ACS Symposium Series No. 221, American Chemical Society, and the references cited therein.
The use of ATBN elastomers having carbon backbones, while increasing toughness, does not provide sufficient thermal and/or oxidative stability for many modern applications of adhesives and matrix resins, particularly those in the aerospace field. Thus it has been proposed to utilize functionalized polysiloxanes for these applications, relying on the thermal-oxidative stability of the silicon-containing backbone to lend increased thermal stability to the total resin system. Several such approaches have been discussed in Riffel, supra, and involve primary amine terminated polysiloxanes such as bis(3-aminopropyl)polysiloxanes and secondary amine terminated polysiloxanes such as bis(piperazinyl)polysiloxanes.
Perhaps due to their lower functionality, the secondary amine terminated, piperazinyl polysiloxanes generally proved to have superior physical properties compared to the primary amine terminated polysiloxanes (tetrafunctional). Unfortunately, these secondary amine terminated polysiloxanes are difficult to prepare.
One preparation of piperazinyl functionalized polysiloxanes involves reaction of 2-aminoethylpiperazine with a previously synthesized carboxy-terminated polysiloxane to form the bis(2-piperazinyl ethyl amide) of the polysiloxane: ##STR1##
A second approach is to react a large excess (to avoid polymer formation) of piperazine with a bis-epoxy polysiloxane, producing a bis(2-hydroxy-3-piperazinyl) polysiloxane: ##STR2## This method, of course, requires prior preparation of the epoxy-functional polysiloxane.
Ryang, in U.S. Pat. No. 4,511,701, prepared both primary and secondary amine-terminated polysiloxanes by reacting an appropriately substituted diamine with difunctional silylnorbornane anhydrides, themselves prepared as disclosed by Ryang in U.S. Pat. No. 4,381,396. Reaction of these diamines with the bis(anhydride) functional polysiloxanes results in amino-imides such as: ##STR3##
Only the last-mentioned process produces amino-functional polysiloxanes which are truly difunctional. The amide hydrogen and hydroxyl hydrogen produced by the first two preparations, though less reactive than the secondary amino hydrogens, are nevertheless reactive species in most resin systems. Their presence, therefore can cause further, and at times unpredictable crosslinking, either over an extended period of time in normal service, or as a result of high curing temperatures.
Furthermore, all of the foregoing preparations involve many steps, and in the process consume large quantities of relatively expensive chemical reagents. All these prior art products are difficult to prepare, expensive products, and thus there remains a need for thermally stable, secondary amine terminated polysiloxanes which may be prepared in high yield and in an economic manner.